Sealants for structural member joints and methods of using same

ABSTRACT

Thermoplastic sealants and methods for forming a joint, such as a weld joint, between one or more structural members using thermoplastic sealants are provided. The thermoplastic sealants have melting temperatures lower than the melting temperature of at least one of the structural members. The thermoplastic sealants may further include fillers, and are disposed between faying surfaces of the structural members. The sealants can fill the spaces between the structural members to prevent the entry of chemicals, moisture, debris and other substances.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with support of the United States Governmentunder Air Force Research Laboratory Contract No. FA8650-04-C-5704. TheGovernment has certain rights in this invention.

FIELD

The present invention relates to structural member joints, and moreparticularly, to sealants for structural member joints and methods ofusing same.

BACKGROUND

Joints between structural members are often formed by overlapping thetwo members together to create a lap joint. The lap joint is securedtogether by various means. In some instances, holes are drilled andrivets or other fasteners are disposed through the holes. Suchoverlapping surfaces can also be joined by welding the structuralmembers together using one or more welds at the overlap of thematerials. One type of welding process used to join lap joints isfriction stir welding (FSW) in which a rotatable pin extends in adirection generally perpendicular to the interface of the members and isurged through the members along the interface.

However, the surfaces of each piece of joined metal adjacent thefasteners or welds mated by the lap joint, called “fay” surfaces, areoften not fully bonded by the weld or other fasteners. The resultingspaces in the interface are open to moisture, chemicals, debris, andother foreign materials. This can result in increased corrosion of thestructural members. In FSW, for example, the resulting nugget istypically not as wide as the interface of the overlapping members, suchthat the members define spaces in the interface in which corrosion canoccur.

In order to control corrosion in these joints, sealants are oftendisposed in the spaces prior to, during, and/or after welding.Conventional sealants are known to reduce moisture which may be trappedbetween the faying surfaces and/or brought in by capillary action. Thesesealants are also useful for reducing mechanical and fatigue problemsresulting from rubbing between the faying surfaces, vibration, and thelike. Conventional faying surface sealants include polysulfide,polythioether, and the like, which are applied to the mating surfacesprior to joining.

However, many conventional sealants are degraded by the heat andmechanical activities involved in the welding process, as well as byvibration during use of the joined component. In some instances, thesealants become loosened from the interface. Specifically, when thesealant is applied before welding, care must be taken to avoidintroducing the sealant into the nugget of the joint because the sealantcan negatively impact the strength and/or corrosion resistance of thejoint. This can involve placing masking tape on the area of thestructural members where the nugget will be formed, disposing thesealant on the structural members, and then removing the masking tape toleave a relatively clean area for forming the nugget. However, such aprocess is time consuming. Additionally, even if such precautions aretaken, some of the sealant can be squeezed into the interface as membersare positioned and urged together to form the joint. The sealant thenmixes with the plasticized material of the joint, thereby reducing thequality of the weld joint.

Therefore, what is needed are improved sealants and methods for forminga corrosion resistant joint in structural members.

SUMMARY

Sealants and methods for forming a joint, such as a weld joint, betweenone or more structural members are provided. In one embodiment, thesealant, which is disposed between faying surfaces of the one or morestructural members, comprises a thermoplastic material having a meltingtemperature lower than the melting temperature of at least one of thestructural members. In embodiments which utilize friction stir welding,the temperature is also preferably lower than the highest temperaturegenerated during friction stir welding. In most embodiments, the sealantfills the spaces between the one or more structural members to prevententry of chemicals, moisture, debris, and other substances, therebyreducing the likelihood of corrosion of the joint or structural membersat the interface. In one embodiment, the one or more structural memberscomprise first and second substantially parallel members defining thefaying surfaces respectively, and the joint extends substantiallyperpendicular through the interface of the faying surfaces. In oneembodiment, at least one of the substantially parallel members has aT-configuration or sub-structure. In one embodiment, there is only asingle structural member such as a tubular member or otherwise curvedmember with overlapping edges.

In one embodiment, the thermoplastic sealant is selected from the groupconsisting of polyethylenes, polypropylenes, polystyrenes,polyvinylchlorides, polytetrafluoroethylenes, polyamides (e.g., nylon-6,nylon-11, nylon-12, nylon-13 or nylon 6/6), acrylics, acetals,polycarbonates, polyesters, further including polyimides,polyetheretherketones, polyphenylene sulfides, polyether sulfones,polyamideimides, polyphenylene oxides, and any combination thereof. Inone embodiment, the thermoplastic sealant contains no fillers. Inanother embodiment, the thermoplastic sealant contains one or morefillers such as plasticizers, glass fibers, coloring additives,conductive additives, metal oxides and any combination thereof. Althoughthe sealant is preferably non-conducting and therefore does not affectgalvanic corrosion of the joint, the invention is not so limited. Inother embodiments, the sealant is conducting, such as with the additionof copper or aluminum powder, such that the sealant can be cathodicrelative to at least one structural member.

The sealant can have any suitable melting temperature. In oneembodiment, the sealant has a melting temperature less than about 500°C. In another embodiment, the sealant has a melting temperature of atleast about 150° C.

In one embodiment, a sealed joint comprising at least one structuralmember defining first and second faying surfaces in an opposedconfiguration and defining an overlapping interface therebetween; asealed joint extending through the overlapping interface and connectingthe first and second faying surfaces of the at least one structuralmember; and a thermoplastic sealant disposed in the interface, whereinthe thermoplastic sealant has a melting temperature lower than themelting temperature of the at least one structural member is provided.

In one embodiment, a weld joint comprising at least one structuralmember defining first and second faying surfaces in an opposedconfiguration and defining an overlapping interface therebetween; a weldjoint extending through the overlapping interface and connecting thefirst and second faying surfaces of the at least one structural member;and a thermoplastic sealant disposed in the interface, wherein thethermoplastic sealant has a melting temperature lower than the meltingtemperature of the at least one structural member, are provided. In oneembodiment, the weld joint comprises a nugget area formed by frictionstir welding. In one embodiment, the sealant substantially fills theinterface.

In most embodiments, each structural member is made from the same metal,although the invention is not so limited. In other embodiments, thereare at least two structural members, each made from a different metal.In yet other embodiments, there are at least two structural members,with at least one structural member made from a metal composite ornon-metal composite, such as a polymer matrix composite, ceramic,graphite reinforced epoxy, and the like.

In one embodiment, a method of sealing a joint comprising disposing athermoplastic sealant on at least one of first and second fayingsurfaces of at least one structural member; positioning the fayingsurfaces in an opposing configuration to form an interface therebetween;and joining the at least one structural member to form a sealed jointextending through the interface and thereby heating the thermoplasticsealant such that the sealant bonds with the at least one structuralmember proximate to the sealed joint, wherein the sealant has a meltingtemperature lower than the melting temperature of the at least onestructural member is provided. In one embodiment, the joining stepcomprises heating the sealant to at least the melting temperature of thesealant. In another embodiment, the joining step comprises heating atleast a portion of the sealant to a temperature no greater than themelting temperature of the sealant wherein the sealant is bonded to theat least one structural member.

In one embodiment, the joining step comprises rotating a pinless tool,i.e., a burnishing tool, and urging the shoulder parallel to theinterface to thereby friction heat the at least one structural memberand cause the sealant to melt, thus forming a faying surface seal andadhesive bond between the overlapped members. In one embodiment, atleast one of the structural members is a non-metal or non-metalcomposite as described above. The pinless tool is placed in contact withthe metal component and generates sufficient heat to melt the sealant.The tool is then urged along the surface to form a bond to thenon-metallic member. In one embodiment, the metal member is a top skinand the non-metal member is located beneath. The non-metal member mayhave a T-substructure, although the invention is not so limited.

In one embodiment, a method of sealing a weld joint comprising disposinga thermoplastic sealant on at least one of first and second fayingsurfaces of at least one structural member; positioning the fayingsurfaces in an opposing configuration to form an interface therebetween;and welding the at least one structural member to form a weld jointextending through the interface and thereby heating the thermoplasticsealant such that the sealant bonds with the at least one structuralmember proximate to the weld joint, wherein the sealant has a meltingtemperature lower than the melting temperature of the at least onestructural member, is provided. In one embodiment, the welding stepcomprises heating the sealant to at least the melting temperature of thesealant. In another embodiment, the welding step comprises heating atleast a portion of the sealant to a temperature no greater than themelting temperature of the sealant wherein the sealant is bonded to theat least one structural member. In one embodiment, the welding stepcomprises rotating a friction stir welding pin extending from a shoulderand urging the pin through the interface to thereby friction stir weldthe at least one structural member.

The thermoplastic sealant can be disposed in any suitable manner, suchas with plasma spraying, electrostatic spraying or with a type of hot“glue gun,” resulting in a sealant having a thickness of between about0.0025 and 0.1 cm. In one embodiment, the thermoplastic sealant is inthe form of a tape that is applied to the faying surfaces. In oneembodiment, the disposing step comprises disposing the sealant on bothof the first and second faying surfaces. In one embodiment, thepositioning step comprises overlapping the faying surfaces to form theinterface having a width about equal to the width of the shoulder.

The thermoplastic sealants described herein have excellent static andfatigue properties at room temperature, providing excellent corrosioncontrol at minimal cost and environmental impact. The sealants have thefurther advantage of not setting or curing immediately, thus providingflexibility during the manufacturing process as to the timing of sealantapplication and welding. Additionally, there is minimal to no adverseimpact on the strength of the welding joint with use of these sealants.The sealants may be even further optimized with additional surfacepreparation, addition of fillers to provide a desired property,adjustment of weld parameters, and so forth.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a simplified perspective view of a conventional friction stirwelding operation for forming a butt weld between two workpieces in oneembodiment of the present invention.

FIG. 2A is a simplified cross-sectional view showing a friction stirwelding tool configured to weld structural members in a T-configurationin one embodiment of the present invention.

FIG. 2B is a simplified cross-sectional view showing a friction toolconfigured to join structural members in a T-configuration in oneembodiment of the present invention.

FIG. 3 is a simplified cross-sectional view showing a weld resultingfrom use of the friction stir welding tool shown in FIG. 2A in oneembodiment of the present invention.

FIG. 4 is a photographic image of a bare friction stir weld joint in oneembodiment of the present invention.

FIG. 5 is an enlarged microstructure image of a retreating side of thefriction stir weld joint as indicated by FIG. 4 in one embodiment of thepresent invention.

FIG. 6 is an enlarged microstructure image of an advancing side of thefriction stir weld joint as indicated by FIG. 4 in one embodiment of thepresent invention.

FIG. 7 is a plot showing force per distance of weld versus displacementfor a bare T-weld with and without sealant in one embodiment of thepresent invention.

FIG. 8 is a photographic image of an anodized friction stir weld jointin one embodiment of the present invention.

FIG. 9 is an enlarged microstructure image of a retreating side of thefriction stir weld joint as indicated by section line 9 in FIG. 8 in oneembodiment of the present invention.

FIG. 10 is an enlarged microstructure image of an advancing side of thefriction stir weld joint as indicated by section line 10 in FIG. 8 inone embodiment of the present invention.

FIG. 11 is a plot showing force per distance of weld versus displacementfor an anodized T-weld with and without sealant in one embodiment of thepresent invention.

FIG. 12 is an enlarged microscopic image of another portion of the weldjoint of FIG. 8 as indicated by section line 12 in FIG. 8 in oneembodiment of the present invention.

FIG. 13 is a simplified schematic of a gripping device for performingstatic tests in one embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the subject matter may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice them, and it is to be understood that otherembodiments may be utilized and that mechanical, chemical, structural,electrical, and procedural changes may be made without departing fromthe spirit and scope of the present subject matter. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of embodiments of the present invention is defined only bythe appended claims.

The Detailed Description that follows begins with a definition sectionfollowed by a brief overview of frictional stir welding (FSW), followedby a description of the embodiments, an example section and a briefconclusion.

Definitions

The term “thermoplastic polymer” or “thermosoftening plastic” or“thermoplastic” as used herein refers to any plastic that can berepeatedly softened upon heating and hardened upon cooling, in contrastto a thermosetting plastic or a thermoplastic elastomer as definedbelow. Thermoplastics are bonded together by Van der Waals forces andgenerally do not undergo cross-linking upon heating. (However, polymerscontaining a high number of unsaturated bonds can react further underexposure to heat or ultraviolet light to produce cross-linking.Therefore, most thermoplastics will resoften and eventually melt to aliquid state when reheated. Thermoplastics are capable of being formedinto a filament in which the structural elements are oriented in thedirection of the fiber axis. Examples include, but are not limited to,polyethylene, polystyrene and polyvinyl chloride (PVC).

The term “thermosetting plastic” or “thermoset resin” as used hereinrefers to any plastic that can be formed into a shape duringmanufacture, but which sets permanently rigid, i.e., fully cures, duringheating. This is due to extensive cross-linking, which occurs uponheating and cannot be reversed by reheating. Therefore, rather thanmelting when reheated, a thermosetting plastic may only soften before itdecomposes by charring and burning.

The term “thermoplastic elastomer” or “elastomer” as used herein refersto a plastic that can be formed into a shape during manufacture, but has“rubber-like” properties which allow it to stretch and return to itsoriginal shape even after it is set. However, an elastomer cannot bereheated to change its shape, and over time, may become rigid withfurther cross-linking upon exposure to heat or ultraviolet light. Athermoplastic elastomer is capable of undergoing vulcanization, i.e.,cross-linking with sulfur), including room temperature vulcanization(RTV). Thermoplastic elastomers are discussed in U.S. Patent Application2004/0173662 to Christner (hereinafter “Christner”). Oftentimes,elastomers are fluorinated to reduce the temperature at which thematerial becomes brittle and glass-like (see discussion offluoroelastomers, i.e., fluoroelastic polymers, in Christner).

The term “nylon” as used herein is a generic name for any long chainsynthetic polymeric amide which has recurring amide groups as anintegral part of the main polymer chain. Examples include, but are notlimited to, nylon-6, nylon-11, nylon-12, nylon-13 and nylon 6/6.

Overview of Friction Stir Welding

Welding techniques typically use elevated temperatures to bond metalsand metal alloys. Friction stir welding (FSW) is a welding techniquewhich allows two components, e.g., sheets, plates, etc., to be joinedtogether in the solid state, i.e., without melting, through use of arotating cylindrical tool with pin. The welds can be made on many typesof materials in any position at welding speeds of up to several inchesper minute.

In the embodiment shown in FIG. 1, a rotating tool 102 having acylindrical portion 103 with a shoulder 104 is moved along a joint lineor interface 108 located between two members 110, 111. The shoulder 104is located at the junction of the cylindrical portion 103 and a probe orrotatable pin (hereinafter “pin”) 112. The pin 112 extends in adirection generally perpendicular to the interface of the members or isurged through the members 110, 111 along the interface 108. Frictionalheat generated by the pin 112 induces gross plastic deformation orsoftening in the members 110, 111, thus allowing the rotating tool 102to be moved along the interface 108 as the plasticized material is beingmixed by the pin 112. Sufficient downward force 114 needs to bemaintained to ensure continuous contact between the shoulder 104 and theinterface 108. As the plasticized material is mixed and forged togetherbeneath the tool, a FSW joint 116 is formed, characterized by a mixedportion usually having a refined grain structure, referred to as anugget 118. In one embodiment, rotation of the shoulder 104 produces asemi-circular striation pattern on the surface as shown in FIG. 1.

The nugget 118 is generally smaller than the width of the shoulder 104and slightly larger than the pin 112. Although the precisemicrostructure of a friction-stir weld depends on many factors, such astool design, rotation and translation speeds, applied pressure,characteristics of materials being joined, and so forth, friction stirwelded materials have a characteristic cross-section as is known in theart and illustrated in FIG. 1. Specifically, the nugget 118 issurrounded by a thermomechanically affected zone (TMAZ) 120 having adifferent microstructure as is known in the art. Slightly further awayis a heat affected zone (HAZ) 122, with unaffected material 124 locatedyet further away from the FSW joint 116.

The opposing sides of the FSW joint 116 also have specific textures. Anadvancing side 128, i.e., the side on which the motion and rotationdirection of the rotating tool 102 are in the same direction, has asharp boundary between the nugget 118 and the TMAZ 120. (See, forexample, FIGS. 6 and 10). In contrast, a retreating side 130, i.e., theside on which the rotation direction is opposite to the motion of therotating tool 102, has a more complex microstructure with no clearboundary between the nugget 118 and the TMAZ 120. (See, for examples,FIGS. 5 and 9).

In the embodiment shown in FIG. 1, the butt weld has a gap 150, i.e.,the weld is incomplete. Such a weld would benefit from application of asealant, such as the sealants described herein.

FSW produces solid state joints without the addition of filler or theuse of shielding gases. This technique allows welding of materials whichwere previously considered difficult to weld reliably without thepresence of voids, cracking or distortion. FSW is advantageously used inmany applications, including, but not limited to, aircraft construction.Other details of FSW are understood in the art and will not be discussedfurther herein. See, for example, “Characterising texture variations ina friction stir welded aluminium alloy,”http://www.hkltechnology.com/data/0-FSW-aluminium.pdf, EBSD ApplicationNotes, HKL Technology, Inc., Danbury, Conn., 2005, pages: 28-31 and U.S.Pat. No. 5,460,317 to Thomas, et al., both of which are incorporatedherein by reference.

Discussion of the Embodiments

In one embodiment, a sealed joint comprising at least one structuralmember defining first and second faying surfaces in an opposedconfiguration and defining an overlapping interface therebetween; asealed joint extending through the overlapping interface and connectingthe first and second faying surfaces of the at least one structuralmember; and a thermoplastic sealant disposed in the interface, whereinthe thermoplastic sealant has a melting temperature lower than themelting temperature of the at least one structural member is provided.In one embodiment, there are two structural members, each formed of adifferent metal. In yet other embodiments, there are two structuralmembers, and at least one structural member is formed of a metalcomposite or non-metal composite, such as a polymer matrix composite,ceramic, graphite reinforced epoxy, and the like.

In one embodiment, a weld joint comprising at least one structuralmember defining first and second faying surfaces in an opposedconfiguration and defining an overlapping interface therebetween; a weldjoint extending through the overlapping interface and connecting thefirst and second faying surfaces of the at least one structural member;and a thermoplastic sealant disposed in the interface, wherein thethermoplastic sealant has a melting temperature lower than the meltingtemperature of the at least one structural member, are provided. In oneembodiment, the weld joint comprises a nugget area formed by frictionstir welding. In one embodiment, the sealant substantially fills theinterface.

FIG. 2A shows a cross section of a lap joint between a first structuralmember 210 and a second structural member 211. In the embodiment shownin FIG. 2A, the first structural member 210 is a top skin and the secondstructural member 211 has a T-substructure, although the invention isnot so limited. The sealants and associated methods described in thevarious embodiments of the present invention can be used on virtuallyany type of interface which can be joined together. In one embodiment,both members are planar, such as the members shown in FIGS. 1 and 2 ofU.S. Pat. No. 6,905,060 to Van Aken. In another embodiment, a singlestructural member is joined using the various methods described herein.The single structural member can include, but is not limited to, atubular member or an otherwise curved member with overlapping edges. Inyet another embodiment three or more structural members are joined. Inone embodiment, the various sealants of the present invention are usedin a butt joint in which a faying surface is created by a nugget thatonly partially joins the materials.

The faying surfaces are defined where the first structural member 210overlaps the second structural member 211. Specifically, the firststructural member 210 has a first faying surface 236 and the secondstructural member 211 has a second faying surface 238, such that aninterface 208 is formed therebetween. (In the embodiments shown in FIGS.2A, 2B and 3, the interface 208 is enlarged for clarity).

Any size and shape of rotating tool 102 can be used. In the embodimentshown in FIG. 2A, the diameter of the cylindrical portion 112 engagedwithin 210 is wider than the diameter of the pin 112 engaged within 211,although the invention is not so limited. The pin 112 of the rotatingtool 102 is inserted through the interface 208 generally perpendicularto the interface 208. An anvil (not shown) or other support can bedisposed against the second structural member 211 to oppose the rotatingtool 102. The rotating tool 102 is then urged against the firststructural member 210 and advanced along the interface 208 of thestructural members 210, 211 as the pin 112 rotates, i.e., into the planeof the paper, with the shoulder 104 of the rotating tool 102 contactingthe first structural member 210. The friction generated by the pin 112causes the material in each member to plasticize as discussed above. Thematerials are then mixed and forged by the shoulder 104 and pin 112 toform a weld joint 316 as shown in FIG. 3. The weld joint 316 connectsthe first and second structural members 210, 211. The weld joint 316 ischaracterized by a nugget 318 proximate to the path of the pin 112 asshown in FIG. 2A.

The weld joint 316 formed by the configuration shown in FIG. 2A isgenerally referred to as a lap joint, i.e., a joint generallyperpendicular to the interface of overlapping members. In otherembodiments, other weld joints can alternatively be formed by frictionstir welding. In addition, other types of friction welding, such aslinear friction welding, friction stir spot welding (see, for example,U.S. Pat. Nos. 6,892,924 and 6,883,699 to Stevenson, et al., both ofwhich are hereby incorporated by reference), and the like, can also beused to join members. In other embodiments, welding devices and methodsother than friction welding devices and methods are used. Further, thestructural members 210, 211 can also be joined without welding, forexample, by solder joints, braze joints, rivets, bolts, clips, otherfasteners, crimps, and the like. Embodiments of the present inventionare not intended to be limited to these or other types of joints, andinstead can be used with a wide variety of joints for connectingstructural members 210, 211.

The structural members 210, 211 can define any of a variety of shapessuch as sheets, plates, blocks, and the like. Essentially, thestructural members 210, 211 can be any structure that can also be joinedtogether by riveting. The members 210, 211 can be formed of metals, suchas aluminum, titanium, or alloys thereof, metal matrix composites, andthe like. In one embodiment, one or both members are made from non-metalcomposites. In one embodiment, the members 210, 211 are anodized, e.g.,anodized aluminum, although anodization is known to reduce the ductilityand strength of a weld, such as a friction stir weld. Further, themembers 210, 211 can be joined to create an assembly used for variousapplications including frames, panels, skins, airfoils, and the like foraeronautical and aerospace structures, such as aircraft and spacecraft,marine vehicles, automobiles, and the like. In some applications, themembers 210, 211 are joined in geometrical configurations that makedifficult or prevent subsequent access for inspecting or treating thejoint. For example, the structural members 210, 211 can be overlappedand joined to form a partially or fully closed body such as an airplanewing. In other embodiments, the structural members 210, 211 can be afuel tank, i.e., a wet bay.

FIG. 2B provides an alternative embodiment in which a pinless tool 202,i.e., a burnishing tool, having a body 103 and a shoulder 104 as in FIG.2A, is used in a non-welding procedure to join together structuralmembers 210, 211 having overlapping joints. The members 210, 211 arepositioned as described herein, and a sealed joint (similar to the weldjoint 316 in FIG. 3) is formed, which extends through the interface 208when the sealant 260 is heated, such that the sealant 260 bonds with oneor more of the structural members (210 or 211) proximate to the sealedjoint. The sealant 260 necessarily has a melting temperature lower thanthe melting temperature of one of the structural members 210, 211. Suchan embodiment is useful when each structural member 210, 211 is madefrom a different type of metal, such as aluminum and steel, or when atleast one of the structural members 210, 211 is a non-metal or non-metalcomposite, such as a polymer matrix composite, ceramic, graphitereinforced epoxy, and the like. In one embodiment, member 210 is themetal member and 211 is the non-metal member. As with the other figures,although 211 is shown having a T-substructure, the invention is not solimited.

Referring again to FIGS. 2A and 3, although the faying surfaces 236, 238of the structural members 210, 211 can correspond closely in contour,the interface 208 is characterized by spaces or voids 250 between thesurfaces 236, 238 where the weld joint 316 is not formed, i.e., oneither side of the weld joint 316. According to one embodiment of thepresent invention, a sealant 260 is disposed between the structuralmembers 210, 211 at the interface 208 thereof prior to the weld joint316 being formed, although the invention is not so limited. The sealant260 can be disposed at any suitable time during the manufacturingprocess. The sealant 260 can also be disposed on one or both of thefaying surfaces 236, 238 of the structural members 210, 211, and can bedisposed over part or all of the area of the interface 208, includingthe nugget region 318. Thus, the sealant 260 can fill the spaces 250between the faying surfaces 236, 238 of the structural members 210, 211.The sealant 260 prevents chemicals, moisture, debris, and othersubstances from entering the spaces 250, thereby preventing corrosion orother damage that can be caused by those substances.

Embodiments of the present invention provide a sealant 260 made from athermoplastic material. The thermoplastic material can include, but isnot limited to, polyethylene, polypropylene, polystyrene,polyvinylchloride, polytetrafluoroethylene (i.e., Teflon®), polyamide(i.e., various nylon materials including but not limited to nylon-6,nylon-1, nylon-12, nylon-13, nylon 6/6, and the like), acrylics such asLucite®, acetals, polycarbonates, polyesters, and so forth. Thethermoplastic preferably flows under the application of heat andpressure as would be experienced during a friction stir welding process,although the invention is not so limited. It is possible that thesealant 260 can be used as the primary bonding agent between thestructural members 210, 211, where a tool is rotated on the surface ofstructural member 210 to generate heat and a compressive pressure toforce 210 onto 211.

The sealant 260 can also include one or more additional materials asfillers in order to provide desired properties. For example, the sealant260 can be modified for high or low temperature use, depending on thefiller used. Addition of a plasticizer produces a more pliable sealant260 which is useful at lower temperatures, i.e., below about 23° C.However, it is possible that addition of a plasticizer may adverselyaffect the bond strength of the sealant to the structural members,although those skilled in the art could likely develop a formulationwhich maximizes pliability while minimizing any adverse affect on bondstrength. Any suitable plasticizer can be used. In one embodiment,Parmerol® UNIPLEX 214 made by Unitex Chemical Corporation having officesin Greensboro, N.C. is used. Addition of glass fibers, i.e., E-glassfibers, to powder coatings can improve creep resistance at highertemperatures, i.e., above about 100° C. Other additives add color (e.g.,titania particles add a white color, graphite particles add a blackcolor, and so forth). Yet other additives are conductive additives(e.g., copper) which change electrical properties by making the sealant260 conductive, as discussed above. Yet other additives, such as metaloxides (e.g., copper oxide) help to reduce biofouling, which isimportant on ship hulls. The copper oxide kills bacterial growths thatare the food source of barnacles. Biofouling is also an issue for wetfuel bays where bacteria grow, die and decompose to form an acidsolution. Copper oxide keeps the bacterial population to a minimum, thusreducing the acid produced with decomposition.

In one embodiment, the sealant 260 has a melting temperature comparableto the temperature of the welding process being used. In one embodiment,the sealant 260 is characterized by a melting temperature less than themelting temperature of at least one of the structural members 210, 211and the highest temperature generated during welding. In this way, thesealant 260 can be bonded to portions of the faying surfaces 236, 238 ofthe structural members 210, 211, including portions of the fayingsurfaces 236, 238 where the structural members 210, 211 are not meltedduring welding. The term “melting temperature,” is meant to refer to atemperature at which the sealant 260 becomes at least partially meltedand sufficiently hot for bonding to the structural members 210, 211. Insome embodiments, the sealant 260 is formed of multiple constituentmaterials, one or more of which can have a melting temperature that isequal to or higher than the melting temperature of the sealant 260and/or the structural members 210, 211. Further, the melting temperatureof the sealant 260 can be lower than the melting temperatures of all ofthe constituent materials. The melting temperature of the sealant 260can be less than about 500° C., for example, between about 350° C. and450° C., which is less than the melting temperature of many aluminum andtitanium alloys and lower than the temperatures at which these materialsare typically friction stir welded. Thus, the melting temperature can besufficiently low so that some or all of the sealant 260 is melted duringthe weld process, for example, by the frictional heat generated duringfriction stir welding. In one embodiment, the sealant 260 has a meltingtemperature no less than about 150° C. In another embodiment, thesealant 260 has a melting temperature between about 150° C. and 180° C.or greater, up to about 200° C. or more. In yet another embodiment, thesealant 260 has a melting temperature range between about 150° C. and180° C.

The sealant 260 is disposed as a polymer layer, i.e., no curing orpolymerization reaction occurs. The sealant 260 also does not undergoany type of vulcanization, such as RTV. Since no curing or vulcanizationtakes place with this material, the sealant 260 can be applied atessentially any stage of the manufacturing process prior to the jointbeing welded. This is in contrast to known sealants, such asthermosetting plastics or thermoplastic elastomers, which are disposedas a monomer layer and then cured to form a polymer. Most thermoplasticelastomers also undergo room temperature vulcanization, thus requiringimmediate welding after application of the sealant.

In one embodiment, the sealant 260 is repairable even after the jointhas been welded. This can be important if it is determined after thefact that the sealant was not disposed in certain areas of the joint,was disposed too thinly, or is just not holding for any reason.Specifically, the sealant 260 can be remelted using any suitable heatsource, such as in an oven, a propane torch, an air heating gun and thelike. The joint can also be repaired by rewelding such that the FSW toolis urged through the joint and the sealant remelted. In one embodiment,the sealant 260 is remelted more than once. In one embodiment, thesealant is repairable by heating the joint to at least the meltingtemperature of the sealant for a sufficient period of time to bond withthe structural members 210, 211. In a particular embodiment, a jointshowing sealant separation is heated to about 200° C. for about 15minutes to repair the joint.

The sealant 260 can be formed by any of various methods that are knownin the art for forming powdered polymer blends, copolymers or mixturesof the aforementioned components together with fillers. For example, thesealant 260 can be formed by melting and mixing the constituentmaterials. The molten mixture can be cast or extruded and allowed tocool to form a solid which can then be milled into a fine powder. Thesealant 260 is then disposed on one or both of the faying surfaces 236,238 of the structural members 210, 211 by any suitable means known inthe art. For example, the sealant 260 can be spread by hand or sprayedas a coating onto the surface, such as with plasma spraying, flamespraying, high velocity oxy-fuel spraying, and the like. In oneembodiment, the sealant is applied using an electrostatic sprayer whichmay provide for a more controlled application of the sealant and auniform thickness, such as about 100 μm. In one embodiment, the sealant260 is applied as a tape, which has the advantages of providingsimplicity of application while avoiding problems associated withoverspray and environmental issues involved in handling and spraying ofpolymer-based powders. In another embodiment, the sealant 260 is appliedusing a type of hot “glue gun,” such that surface preparation steps,such as grit-blasting, may be omitted.

The particles of the sealant 260 can be heated and melted during thedeposition process, while the structural members 210, 211 remainunmelted. Alternatively, the particles can remain unmelted duringdeposition as in electrospray deposition. The amount of sealant 260disposed on the faying surfaces 236, 238 can vary, but in oneembodiment, the layer of sealant 260 is about 0.1 to 0.2 mm (about 0.004to 0.008 in) in thickness. The sealant 260 can be disposed over all orpart of the faying surfaces 236, 238, including the portion of theinterface 208 that is welded to form the nugget area 318 of the joint316, although in some cases the sealant 260 may positively or negativelyaffect certain mechanical properties of the joint 316. Generally, plasmaspraying is preferred for thicker layers. Electrostatic spraying canalso be used, but generally produces thinner layers and is typicallyaffixed by application of heat by any suitable means, such as withfriction stir welding or oven-baking at the melting temperature of thethermoplastic.

The sealant 260 is bonded to both of the structural members 210, 211. Insome embodiments, adhesion of the sealant 260 to the surface is improvedthrough some type of roughening process known in the art, although theinvention is not so limited. In a particular embodiment, one or moresurfaces are abraded by grit blasting prior to sealant application toimprove sealant adhesion. Other surface roughening techniques include,but are not limited to, sandpaper, etc.

The sealant 260 is then heated to a temperature sufficient to thermallybond the sealant 260 to the faying surfaces 236, 238 of the structuralmembers 210, 211, e.g., the melting temperature of the sealant 260. Thethermal energy for heating the sealant 260 can be generated entirely bythe welding process. For example, if the structural members 210, 211 arefriction stir welded, the frictional heat resulting from the motion ofthe pin 112 and the shoulder 104 can heat the structural members 210,211 and the sealant 260, thereby melting the sealant 260 so that thesealant 260 flows fluidly, adheres to the faying surfaces 236, 238, andis bonded thereto. While the sealant 260 close to the path of the pin112 can be heated primarily by the pin 112, the sealant 260 that isfurther from the pin 112 may be heated to a greater extent by frictionalheat generated between the shoulder 104 and the first structural member210 as the shoulder 104 rotates against the structural member 210.Advantageously, the diameter of the shoulder 104 can be increased sothat the shoulder 104 generates frictional heat over a greater area ofthe structural member 210. For example, according to one embodiment ofthe present invention, the diameter of the shoulder 104 is about equalto the width of the interface 208, such that the shoulder 104 generatesfrictional heat over the width of the interface 208. In one embodiment,the sealant 260 is disposed according to the methods described in U.S.Pat. No. 6,045,028 to Martin, et al., which is incorporated herein byreference.

Other welding or other connection methods can also sufficiently heat thesealant 260 to create the bond between the sealant 260 and the fayingsurfaces 236, 238. In addition, a heat source (not shown) other than thedevice used for connection can be provided. For example, a radiantheater, such as an electric or gas oven or other heater, can be used toheat the sealant 260. Alternatively, the heat source can be a laser thatis used to direct light onto the sealant 260 to heat the sealant 260.The laser can be configured to direct the light on the structuralmembers 210, 211 or in a direction generally parallel to a plane of theinterface 208 and toward the edges of the interface 208 to heat thesealant 260 at the perimeter of the interface 208. Such heat sources canalso be used to repair the sealant 260, if needed, as discussed above.

In some embodiments of the present invention, some or all of the sealant260 is not heated to the melting temperature, and therefore does notmelt during the friction welding process. However, when subjected tosufficient heat and pressure during welding, the sealant 260 can bebonded at a temperature less than the melting temperature of the sealant260. Thus, the sealant 260 can bond to the faying surfaces 236, 238 ofthe structural members 210, 211 and seal the interface 208 therebetween.

It is believed that the sealant 260 may reduce the displacement tofailure, i.e., lower ductility, of a bare weld joint 316 by about 35%,and of an anodized weld joint 316 by about 25%. Anodization of thestructural members alone was determined to reduce the displacement tofailure by about 45%. It was also shown (See FIGS. 7 and 11) that thesealant 260 increases the stiffness of both bare and anodized weldjoints 316 by about 25%.

The invention will be further described by reference to the followingexamples, which are offered to further illustrate various embodiments ofthe present invention. It should be understood, however, that manyvariations and modifications may be made while remaining within thescope of the present invention.

EXAMPLE 1

Starting Materials:

Nylon based powders were obtained from Morton Powder Coatings, adivision of Rohm and Haas, having offices in Reading, Pa. The choice ofpowders was based upon powder size and melting temperature. A powdersize of approximately 50 μm was desired for plasma spray application. Amelting temperature less than 356° F. (180° C.) for the thermoplasticsealant was thought to be appropriate for complete sealing of the fayingsurfaces in a T-joint configuration. Powder coatings selected were basedupon nylon-11, a polyamide resin, with the trade name Corvel®. Corvel®White (Morton Powder Coatings product number 78-1001) has a volumeweighted mean diameter of 62 μm and contains both amorphous silica (1 to5%) and titania (5 to 10%) as fillers for color and to promote flow.Corvel® Black (Morton Powder Coatings product number 78-7001) has avolume weighted mean diameter of 57 μm and contains both graphite (1 to5%) and limestone (5 to 10%) as fillers for color and to promote flow.The glass transition temperature for this polyamide is approximately 42°C. (108° F.) and the melting temperature is approximately 150° C. (302°F.). These powders are designed for electrostatic spray application ontoa surface prepared with a resin primer. The coating is then oven-bakedto produce a glossy finished surface.

Test Procedure and Equipment:

The structural members used in these tests were conventional standardaluminum grade bare T-rails provided by Boeing Co. having offices inSaint Louis, Mo. The members were made from a cast aluminum alloy (A357alloy) substructure and bare aluminum sheet (Aluminum alloy 2024-T8).

The two members were positioned to create a faying surface. Surfacepreparation involved grit blasting the faying surface of theT-substructure with silica sand prior to plasma spraying the nyloncoatings onto the surface to enhance bonding of the coating to thesurface.

A Sulzer-Metco 9 MB plasma gun made by SulzerMetco having offices inWestbury, N.Y., was utilized to apply approximately 200 to 300 μm thickcoatings of the sealant. Spray parameters were fixed for all coatingsmade in this experiment at nine (9) kW input energy, 2.3 m³/hr (about 80ft³/hr) argon flow rate, one (1) gram/min powder flow rate for plasmaspraying the Corvel® White and Corvel® Black powders, and a workingdistance of about 7.6 to 10.2 cm (about three (3) to four (4) in).

After being sprayed, the skin was friction stir welded to the T-railwith a Cincinnati Milacron 20V system made by Cincinnati Milacron havingoffices in Cincinnati, Ohio, equipped with an axial force actuator madeby Manufacturing Laboratories, Inc. having offices in Gainesville, Fla.The force actuator has features as described in U.S. Pat. No. 6,050,475to Kindred, which is incorporated herein by reference. The FSW tool thatwas used was designed by Boeing Co. having offices in Saint Louis, Mo.,for use with lap joints formed with material having a thickness of about0.32 cm (about 0.13 in) was used. Prior to welding, however, a 2.54 cm(one (1) in) wide slot was milled in the top surface of each skin toreduce the sheet thickness at the weld to about 0.28 cm (about 0.11 in)to ensure pin penetration past the sealant and into the T-rail. Thefollowing friction stir welding parameters were used: 900 RPM, 25.4centimeters per minute (10 in/min), and 6.2 kN (1400 lbs) of Z-force,i.e., a downward force transmitted through the tool which forced member210 down onto member 211.

Static strength tests were conducted on sealed joints containing eitherCorvel® black or Corvel® white. Static tensile strengths were measuredusing a MTS Model 312.31 test frame equipped with a 100 kNservohydraulic actuator manufactured by MTS Systems Corporation havingoffices in Eden Prairie, Minn., equipped with an Instron 8800 digitalcontrol and data acquisition system made by Instron Corporation havingoffices in Norwood, Mass.

Test specimens were made from members 210 and 211, respectively, andwere cut to provide approximately 3.81 cm (1.5 in) of welded joint. Eachspecimen was oriented so that the “T” substructure was in an invertedposition. Prior to testing, the specimens 210 and 211 were preloaded to0.44 to 0.89 kN (about 100 to 200 lb_(f)) to prevent the wedge gripsfrom slipping. Tests were run in displacement control with a ramp rateof approximately 0.02 mm/sec (about 0.05 in/min).

A gripping fixture having many of the components as shown in FIG. 13 wasused in these tests. For example, ear clamps 1306 as shown in FIG. 13were used to hold test specimen 210 for the static tests. However,rather than using a stationary clamp 1302 as shown in FIG. 13, an MTSAdvantage TM wedge grip manufactured by MTS Systems Corporation was usedto hold test specimen 211. In tightening the wedge grip, a tensilepreload was applied to the test specimen. (However, as described inExamples 2 and 3 below, such preloading was not necessary with thestationary clamp 1302 of FIG. 13). During this testing, the ear clamps1306 were separated by a distance 1307 of about 3.7 cm (about 1.47 in).Each ear clamp 1306 was secured in place to a clamping platen 1308 withtwo Grade 8 steel fasteners 1303 (one shown for each ear clamp 1306).The clamping platen 1308 is attached to a hydraulic piston (not shown)via a clamping platen threaded member 1310. The clamping platen 1308 wasused to induce a downward displacement of test specimen 210 as is knownin the art.

Welding Procedure Following Application of Corvel® Black:

The first approximately 38.1 cm (15 in) of the faying surface was weldedat about 900 rpm and 25.4 cm/min (10 in/min). The feed rate wasdecreased to about 15.2 cm/min (about six (6) in/min) for the second38.1 cm to provide a hotter weld. A 5.1 cm (two (2) in) gap in theCorvel® Black layer was present in the second half. Constant force wasmaintained throughout the weld, with the FSW tool dipping about 0.05 mm(about 0.002 in) in the area of the gap. During the welding process, theCorvel® Black melted, forming an ideal fillet everywhere except for theinitial portion of the weld and the region originally void of sealant.At the beginning of the weld the skin did not contact the casting, butas the Corvel® Black melted, the skin was pushed down into directcontact with the T-rail. This is likely due to displacement of aluminumfrom the aluminum sheet (2024-T8) into the gap.

Welding Procedure Following Application of Corvel® White:

The baseline parameters described above were intended for the entireweld. However, as the tool travel range was not correctly set, thedesired Z-force was not imposed for the first part of the weld. Thedesired Z-force was obtained from about 53.3 to 76.2 cm (about 21 to 30in) and a sound weld was produced in this section. The results reportedherein are from this section of the weld.

FIG. 4 is a photographic image taken by a Sony CCD video camera, modelno. XC-77, made by Sony Corporation, having offices in Tokyo, Japan,together with a Nikkor 55 mm lens made by the Nikon Corporation, alsohaving offices in Tokyo, Japan. FIG. 4 shows a resulting weld nugget 318between structural members 210 (bare top skin), 211 (bareT-substructure) which contains sealant 260 (Corvel® White). The weldnugget 318 is located between the retreating side (shown in more detailin FIG. 5) and the advancing side (shown in more detail in FIG. 6). Thegap 250 is also visible in FIGS. 5 and 6, with the sealant 260 visiblein FIG. 6. There is little evidence of sealant intrusion into the weldnugget 318.

Results:

The average results of testing on bare aluminum with specimenscontaining sealant are summarized in Table 1 below:

TABLE 1 Summary of static tensile tests for friction stir weldedT-Joints on Bare Aluminum *Maximum load *Sealant failure load ConditionkN/cm (lb_(f)/in) kN/cm (lb_(f)/in) Bare 2.94 ± 0.05 (1679 ± 31) 2.94 ±0.14 2.5 ± 0.33 Corvel ® White (1678 ± 80) (1424 ± 187)  2.4 ± 0.14 0 to1.45 Corvel ® Black (1372 ± 79) (0 to 828)  *Uncertainty is based uponone sample standard deviation. All joints tested were welded at a tooltraverse speed of 25.4 cm/min (10 in/min) and a tool rpm of 900.

A comparison of loading curves “A” and “B” for a Corvel® White sealedjoint and unsealed joint, respectively, is shown in FIG. 7. As FIG. 7shows, the Corvel® White sealant performed very well, with staticstrengths equivalent to the bare metal welded joints. The sealed joint(curve “A”) has a greater stiffness, which indicates that the sealant iscarrying a portion of the applied load. This is corroborated by thesudden load drop each time the sealant fails. Specifically, the firstload drop 702 corresponds with failure of the sealant in the advancingside of the weld (such as is shown in FIG. 6). The second load drop 704corresponds with failure of the sealant in the retreating side of theweld nugget itself. After the second load drop 704, curves “A” and “B”follow nearly the same path. The test ends with failure of the weldnugget itself.

In all instances, there was an initial sealant failure at a load lessthan maximum, i.e., the sealant in one of the faying surfaces broke at75 to 90% of the maximum load. Although FIG. 7 shows the advancing sidefailing first, there appears to be no correlation between sealantfailure and the retreating or advancing side of the weld in thisexample. It is thought that this phenomenon may be related to the toolpushing sealant from the advancing side and depositing it on theretreating side in the same manner it moves the metal during welding.Thus, the retreating side tended to have a well-sealed joint while theadvancing side tended to lose sealant. Application of additional sealanton the advancing side may alleviate this problem as any excess sealantwould be expelled out of the gap by the downward Z-force. Additionally,close examination of the T-rail member indicates some asymmetry in thecross-section, which may have induced a bending moment as the joint wasloaded in tension. Bending may also have been induced by smallmisalignments in gripping the specimens.

In at least two other tests with comparable results the second sealedsurface failed simultaneously with the weld nugget. Although this hasnot yet been observed during testing, it is also possible that the firstsealed surface may fail simultaneously with the weld nugget or that allthree areas (retreating side, weld nugget and advancing side) may failsimultaneously.

The Corvel® Black did not effectively bond to the aluminum top skin.Sealant failure was substantially lower than joints sealed with Corvel®White as shown in Table 1. It is very likely that the graphite inCorvel® Black was welded at too low of a temperature, thus causing poorbonding. It is also possible that surface preparation would improvebonding. (Subsequent testing has shown that higher welding temperaturesproduced by increasing the tool RPM to about 1000 to 1100 RPM, ordecreasing the weld in speed to about 18 to 23 cm/min, or a combinationof both, improve the bonding of the sealant).

EXAMPLE 2

The same starting materials, test procedures and equipment as describedin Example 1 were used in Example 2, except that Corvel® Black was nottested and the top plate was not milled to reduce the thickness at theweld. Also, varying tool RPMs were used as shown in Table 2 and twodifferent tool traverse speeds were used, also as shown in Table 2.Additionally, members were anodized using a standard sulfuric acidanodization as is known in the art. (See also Military SpecificationMil-A-8625).

FIG. 13 shows the custom-made gripping fixture 1300 used to hold bothtest specimens 210 and 211 for these tests. The test specimens 210 and211 were gripped in three locations as shown in FIG. 13, with the weldnugget 318 located centrally between ear clamps 1306 as shown. However,unlike the arrangement used in Example 1, stationary clamp 1302 was usedto secure structural member 211) in place during testing. The clamp 1302was secured in place with two Grade 8 steel fasteners 1303 (one shown).The stationary clamp 1302 further has a stationary clamp threaded member1304 for attachment to a load cell (not shown), which is a component ofthe servohydraulic test frame noted above, as is known in the art. Theear clamps 1306 were used to affix structural member 210 (the “topskin”). During this testing, the ear clamps 1306 were separated by adistance 1307 of about 3.7 cm (about 1.47 in). As noted above, each earclamp 1306 was secured in place with two Grade 8 steel fasteners 1303(one shown for each ear clamp 1306). Test specimen 210 rests on theclamping platen 1308 as described above. (The components of the grippingfixture 1300 can be altered and may vary during future testing. Forexample, the ear clamps 1306 may be separated a greater or lesserdistance than distance 1307 shown in FIG. 13, the fasteners 1303 may bemade from any suitable material other than Grade 8 steel, and so forth).

FIG. 8 is a photographic image of the welded joint taken by the sametype of camera and lens as described in Example 1, showing a weld nugget318 between structural members 210 (anodized top skin), 211 (anodizedT-substructure) containing sealant 260 (Corvel® White). Although notvisible in these images, friction stir welding through the anodizedaluminum leaves oxide debris on the retreating side of the weld nuggetthat is not otherwise present after friction stir welding of barealuminum. The weld nugget 318 is located between the retreating side(shown in more detail in FIG. 9) and the advancing side (shown in moredetail in FIG. 10). Sealant 260 was observed to fill the faying surfacegaps on both sides of the weld nugget 318. However, there is littleevidence of sealant intrusion into the weld nugget 318, and the force ofthe welding process appears to have displaced the sealant from thefaying surfaces next to the weld nugget 318 on both the retreating (FIG.9) and advancing (FIG. 10) sides of the weld, as no sealant is visiblein FIG. 9 and only minimal sealant is visible in FIG. 10. Some voidswere present in the sealant 260, but did not appear to be connected. Thetop skin anodization layer 902 and the T-substructure anodization layer903 are visible in FIG. 9. These anodized layers are compressed togetherthrough the force of the friction stir welding process such that thesealant 260 was excluded and the gap 250 is nearly closed. Compressedanodized layers 902 and 903 are also visible in FIG. 10 adjacent to theweld nugget. Areas of porosity 1010 in the aluminum casting of member211 are also apparent in FIG. 10, although these are common in suchmaterials and do not affect the results herein.

Results:

The average results of testing on anodized aluminum with specimenscontaining sealant are summarized in Table 2 below:

TABLE 2 Summary of static tensile tests for friction stir weldedT-Joints on anodized aluminum Tool *Max Load *First Sealant failureTraverse Speed Tool rpm kN/cm (lb_(f)/in) kN/cm (lb_(f)/in) 21.6 cm/min900 2.2 ± 0.17 0.7 ± 0.23 (1240 ± 97) (393 ± 130) 1000 2.1 ± 0.06 0 to0.93 (1188 ± 37) (0 to 533)  1100 2.3 ± 0.09 0 to 1   (1315 ± 50) (0 to575)  25.4 cm/min. 900 2.5 ± 0.02 (1583 ± 13) no sealant 900 2.5 ± 0.07(1436 ± 38) no sealant 900 1.9 ± 0.08 0 to 0.89 (1092 ± 43) (0 to 510) 1000 weld was not good 1100   2 ± 0.16 0 to 0.53 (1149 ± 90) (0 to 303) *Uncertainty values were calculated as one sample standard deviation.

As Table 2 shows, the average maximum load for anodized aluminum with nosealant varied between sets of specimens. This is thought to be due tothe variance in anodization thickness between samples, with higherlevels of anodization corresponding with reduced maximum loads.

A comparison of loading curves “A” and “B” for an anodized Corvel® Whitesealed joint and an anodized unsealed joint, respectively, is shown inFIG. 11. As FIG. 11 shows, the sealed joint (curve “A”) has a greaterstiffness, which is corroborated by the sudden load drop each time thesealant fails. Specifically, the first load drop 1102 corresponds withfailure of the advancing side of the weld (such as is shown in FIG. 9).The second load drop 1104 corresponds with failure of the sealant on theretreating side of the weld. After the second load drop 1104, curves “A”and “B” essentially follow the same path. The test ends with the failureof the weld nugget itself.

Overall, however, these results are not as good as the results shown inFIG. 7 for bare aluminum. This is not surprising given the known adverseimpact of anodization on ductility and strength of the welded joint.Additionally, there is some concern that use of a new grippingarrangement for the cast A357 T-substructure may have affected theresults. It is possible that a slight bending moment on the weld madethe load more severe on the stronger side, i.e., the advancing side.Additional testing as described in Example 3 appears to confirm thistheory.

FIG. 12 is an image taken where indicated in FIG. 8, i.e., to the farside of FIG. 9 (advancing side). FIG. 12 shows a portion of theT-substructure 211 after the mechanical test where the top skin 210 (notshown) was peeled away, causing failure of the seals and the weldnugget. The T-substructure anodization layer 903 can be seen on thebottom side of the sealant 260. Presence of the top skin anodizationlayer 902 on top of the sealant 260 indicates that the cause of the loaddrop (1102) shown in FIG. 11 was due to the separation of the top skinanodization layer 902 from the top skin member 210 (not shown), i.e.,the top skin anodization layer 902 failed prior to the sealant 260during these tests. Specifically, anodization decreases the loadcarrying capacity of the sealant, and therefore the amount ofdisplacement to failure. FIG. 12 further shows an epoxy layer 1204 formounting the surface for imaging.

EXAMPLE 3

The same starting materials, test procedures and equipment as describedin Example 2 were used in Example 3, although all welding was done at21.6 cm/min (about 8.5 in/min) with RPMs varying between 1000 and 1100as shown in Table 3. Additionally, members were anodized using astandard sulfuric acid anodization as is known in the art. Again,Corvel® White was used as the sealant.

These tests sought to determine the relationship between the fixture1300 and clamping of the advancing side and a determination of whichside, i.e., advancing or retreating, failed first.

Results:

Table 3 provides the results of static tensile tests for friction stirwelded T-joints on anodized aluminum:

TABLE 3 Summary of additional static tensile tests for friction stirwelded T-Joints on anodized aluminum *Max Load *First Sealant failureTool rpm kN/cm (lb_(f)/in) kN/cm (lb_(f)/in) 1000 2.3 ± 0.13  0.97 to0.41 (1298 ± 77) (552 ± 234) 1100 2.3 ± 0.096 0.79 ± 0.29 (1328 ± 55)(453 ± 168) *Uncertainty values were calculated as one sample standarddeviation.

The above values each represent average values from sixteen differentspecimens tested at two different RPMs. From each group of eight, fourspecimens were tested with the advancing side under one of the earclamps and four were tested with the advancing side under the other earclamp. During testing, it was noted which seal broke first and whichclamping side broke first.

Relative to the clamping, 62.5% of the sixteen seals failed on one earclamp first, again with half of the specimens tested with the advancingside under this ear clamp and half of the specimens with the advancingside under the other ear clamp. Thus, it appeared that alignment wasrelatively good. 75% of the weld seals failed on the retreating sidefirst and 25% failed on the advancing side first, regardless of whichear clamp the advancing side was gripped with. Thus, it does appear thatthe retreating side is weaker than the advancing side. These results areconsistent with what would be expected, since the weld nugget is knownto be weaker on the retreating side.

These slightly improved tested results as compared with Example 2 aretherefore thought to be due to improved alignment of the test structureshown in FIG. 13 with the various specimens tested. It is believed theseresults are more representative of the actual tensile strength of thespecimens due to the improved test methods.

EXAMPLE 4

The same starting materials and test equipment as described in Example 3were used in Example 4. Welding was performed with and without sealantto produce test specimens for cyclic fatigue tests. Welding parameterswere chosen such that the static strength of the welded T-joint withoutsealant was the highest average value and the weld parameters for theT-joint with sealant produced the highest static strength for the firstsealant failure. Again, Corvel® White was used as the sealant.

These tests sought to determine the relationship between the sealant andcyclic fatigue life of the welded T-joint. Fatigue tests were conductedusing the gripping fixture shown in FIG. 13. Fatigue tests wereconducted in load control where the mean load and cyclic amplitude wereprogrammed to produce an R ratio of 0.1, i.e. minimum load divided bymaximum load is equal to 0.1. The maximum load was set at 0.79 kN per cmof weld (450 lb_(f)/in) and cycled at approximately eight (8) Hz using asinusoidal waveform. It was expected that some of the sealed jointswould fail at the beginning of the test since the maximum load is closeto the average reported in Table 3.

Results:

Table 4 provides the results of fatigue tests for friction stir weldedT-joints on anodized aluminum:

TABLE 4 Summary of fatigue tests for friction stir welded T-Joints onanodized aluminum Tool Traverse Speed Tool rpm *Number of Cycles toFailure Without sealant 25.4 cm/min. 900 103,000 ± 32,500 With sealant21.6 cm/min 1000 203,000 ± 83,800 *Uncertainty values were calculated asone sample standard deviation.

The above values each represent average values. There were eightspecimens tested that were welded without sealant and nine specimensthat were welded with sealant. During testing, it was noted that morethan 50% of the specimens with sealant had a sealant failure either onthe advancing side or the retreating side during the first 1000 cycles.In one test, the sealant remained intact for more than 65,000 cycles andthat specimen failed after 299,000 cycles.

The improved fatigue life of the welded T-joints containing the sealantis believed to result from the sealant bonding the faying surfacestogether which increases the stiffness of the joint and reduces theopening of the faying surface gap 250 next to the weld nugget.

CONCLUSION

Nylon-11 appears to be an effective sealing agent for friction stirwelded lap joints, although the invention is not so limited. It islikely that other nylons, as well as other thermoplastics, will performat least as well, if not better. However, the low concentration ofunsaturated bonds present in certain thermoplastics (such as nylon-11)may be desirable in limiting cross-linking during exposure toultraviolet radiation. Additional testing can determine the effect ofunsaturated bond concentration on cross-linking, as well as thecorrelation between sealant strength and friction stir weldingparameters. In other embodiments, welding other than friction stirwelding may be used. In yet other embodiments, means for joining twosurfaces together, other than welding, may also be used in combinationwith the thermoplastic sealants described herein.

Although testing herein was performed with plasma spraying, it isexpected that electrostatic spraying will provide for increased controlin application of the sealant. Additional testing with tapes and othermeans of disposing the sealant may also be performed.

The thermoplastic sealants described herein have excellent static andfatigue properties at room temperature. They provide excellent corrosioncontrol at minimal cost and environmental impact. Unless conductivefillers are added as an option to provide a desired feature, thesealants are chemically inert relative to the corrosion of the metallicstructure and therefore do not induce galvanic corrosion. The sealantsdo not undergo a “curing” step or room temperature vulcanization, thusproviding flexibility during the manufacturing process as to the timingof sealant application and welding. Additionally, there is minimal to noadverse impact on the strength of the welding joint with use of thesesealants, and the sealed joints actually exhibit a higher jointstiffness as compared with a welded joint having no sealant, thuspossibly protecting the weld nugget itself from fatigue damage. Thesealants are also repairable as needed. The sealants may be even furtheroptimized with additional surface preparation, addition of fillers toprovide a desired property, adjustment of weld parameters, and so forth.Additionally, copolymerization or blending of multiple types ofthermoplastics, or a combination of copolymerization and blending, mayprovide additional advantages. For example, a higher temperature sealantmay be obtained by blending nylon-11 with nylon 6/6. Similarly, improvedductility or low temperature properties may be enhanced by the additionof polyethylene or a plasticizer to nylon-11.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which thisinvention pertains, having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A joint usable for aeronautical and aerospace structures comprising:at least one metallic structural member defining first and second fayingsurfaces in an opposed configuration and defining an interfacetherebetween, the interface containing one or more spaces; a weld nuggetarea extending through the interface and connecting the first and secondfaying surfaces of the at least one structural member, wherein the oneor more spaces are located on either side of the weld nugget area; andan adhesive bond located proximate to the weld nugget area in the one ormore spaces of the interface, wherein the adhesive bond comprises athermoplastic sealant having a melting temperature lower than a meltingtemperature of the at least one structural member wherein the sealant ischemically inert relative to the corrosion of the at least one metallicstructural member, does not induce galvanic corrosion and does notundergo a curing step or room temperature vulcanization duringmanufacture of the joint.
 2. The joint of claim 1 wherein the adhesivebond improves fatigue life of the joint as compared to a joint havingonly a weld nugget area by bonding the first and second faying surfacestogether to increase stiffness of the joint and by reducing a fayingsurface gap opening next to the weld nugget area.
 3. The joint of claim1 wherein the polyamide is nylon.
 4. The joint of claim 3 wherein thenylon is nylon-11.
 5. The joint of claim 1 wherein the thermoplasticsealant contains one or more fillers.
 6. The joint of claim 5 whereinthe one or more fillers comprise amorphous silica.
 7. The joint of claim5 wherein the one or more fillers are selected from plasticizers, glassfibers, coloring additives, and combinations thereof.
 8. The joint ofclaim 1 wherein the joint is a lap joint.
 9. The joint of claim 1comprising two metallic structural members.
 10. The joint of claim 1comprising two structural members, each made from a different metal. 11.The joint of claim 1 herein the at least one structural member comprisesfirst and second substantially parallel members defining the first andsecond faying surfaces respectively, wherein the joint extends in asubstantially perpendicular direction through the interface of the firstand second faying surfaces.
 12. The joint of claim 11 wherein the secondsubstantially parallel member has a T-substructure.
 13. The joint ofclaim 1 wherein the thermoplastic sealant is a polyamide, polyimide,polyetheretherketone, polyphenylene sulfide, polyether sulfone,polyamideimide, polyphenylene oxide, or any combination thereof.